It measures some 30mm across the widest section (base) and is some 27mm long. It has a 3/4" BSP female threaded section on the lower end (measuring 1" across in diameter inside), and a 1/2" BSP male threaded one on the top section (measuring 3/4" in diameter outside).

It is used to convert a 3/4" BSP male connection to a 1/2" BSP male one.